Metal containers, particularly aluminum cans, are commonly used as containers for a wide variety of products. After their manufacture, the cans are typically washed with acidic or alkaline cleaners to remove aluminum fines, forming lubricants, and other contaminants. A clean and stain-free aluminum surface is essential to the proper application of inks and other coatings. One undesirable result of the aforementioned cleaning process is that water often is retained on the clean, rinsed metal can surfaces. This represents a major heat load for a dry off oven. A water load reduction is desirable because such a reduction reduces energy requirements and provides faster drying times resulting in an increased production rate, and allowing for the use of thinner gauge aluminum.
It is also desirable that the aluminum cans move smoothly through the conveyor system and onto or off the printer mandrels during the manufacturing process. The term "mobility" is used in the industry to refer to the ability of aluminum cans to travel unimpeded (i.e. sliding smoothly against one another) through the manufacturing process conducted at the highest line speed possible. Cans characterized as having poor "mobility" generally have higher coefficients of static friction.
Thoroughly cleaned aluminum cans, however, are typically characterized by high surface roughness, resulting in a high coefficient of static friction, typically about 1.6. This property hinders the mobility of cans through single filers and printers when attempting to increase line speed. This can result in production losses due to frequent jamming, printer misfeeding problems, and a high rate of can rejects.
Periodically during the can manufacturing process, there are, for various reasons, line stoppages. During such a stoppage, a portion of the cans in the washing cycle can be halted in the dry off ovens where temperatures are sufficiently high to volatilize some of the conventional mobility enhancers from the outer surfaces of the cleaned cans. This can result in a portion of the cleaned cans having increased coefficient of static friction values and, hence, poor mobility.
Thus, a need has arisen in the aluminum can manufacturing industry to modify the coefficient of static friction on the outside surface of the cans to improve their mobility without adversely affecting the subsequent application of printing inks and other coatings. Moreover, the material used to promote enhanced mobility must be oven-stable, that is, it must not volatilize at temperatures encountered in the dry-off ovens. Increased production demands are prompting can manufacturers to increase line and printer speeds to produce more cans per unit time. Thus, improving the mobility of aluminum cans is the general trend in the industry.
U.S. Pat. No. 4,599,116 describes an alkaline cleaning process for aluminum container surfaces. The aqueous alkaline cleaning composition contains an alkalinity agent, a complexing agent to chelate at least some of the metal ions removed from the metal surface by the cleaning solution, and at least one surfactant to remove organic soils from the surfaces of the container and to inhibit white-etch staining of the surfaces. The reference indicates that following cleaning a mobility enhancing conversion coating can be applied to the surface of the can for improving the ability of the cans to move throughout the trackwork.
U.S. Pat Nos. 4,859,351; 4,944,889; 5,030,323; 5,064,500; and 5,080,814 describe lubricant and surface conditioners for application to aluminum cans. These patents indicate that the disclosed compositions reduce the coefficient of static friction on the outside surface of the cans which enhances mobility, thereby permitting an increase in production line speed. The lubricant and surface conditioners disclosed in these patents are water-soluble alkoxylated surfactants, namely, organic phosphate esters; alcohols; fatty acids including mono-, di-, tri-, and poly-acids; fatty acid derivatives such as salts, hydroxy acids, amides, esters, ethers and derivatives thereof and mixtures thereof. The references state that the lubricant and surface conditioner may be applied to the cans during the wash cycle, during one of the treatment cycles, during one of the rinse cycles or after the final water rinse. Both acidic and alkaline wash cycles are disclosed.
U.S. Pat. No. 5,061,389 discloses a composition and process for reducing the coefficient of friction on the surface of formed metal structures, such as aluminum cans, by lubricating the surface with a blend of a polyethylene glycol ester and a fluoride compound.
U.S. Pat. No. 5,286,300 discloses a process and a composition useful as a rinse aid for metal surfaces and for improving mobility of formed metal surfaces comprised of a nonionic polyoxyalkylene glycol and an alkoxy derivative of a nonionic ethoxylated and propoxylated glycol. The reference indicates that the process results in an improvement in the drainage of water from metal surfaces and a reduction in the coefficient of friction of the metal surfaces for improved mobility.
U.S. Pat. No. 5,634,986 discloses a process for improving the mobility of formed metal surfaces and for reducing the metal exposure rating of the surfaces which are subsequently coated. Specifically, the reference teaches contacting the metal surfaces with an aqueous composition comprised of a polyethylene to provide improved mobility and a reduction in metal exposure ratings of subsequently coated metal surfaces. It has been found, however, that use of this process brings about an adverse effect on adhesion of a subsequently applied pigmented basecoat.
U.S. Pat. No. 5,746,837 discloses an aqueous composition for enhancing the mobility of an aluminum can that is transported along a conveyor or trackwork, the composition comprising a carboxylic acid or acid-producing compound with ammonia, an amine or an alkali or alkaline earth metal. The reference also discloses a process for mobility enhancement by applying the aforementioned composition to the outer surfaces of an aluminum can during various stages of the can cleaning process. Additionally, the reference teaches that aqueous compositions of the invention also permit a reduction in can drying temperatures.
Although each of the aforementioned compositions and processes provides improved mobility of formed metal substrates, particularly aluminum cans, the increased demands for faster line speeds and increased production mandates an even further reduction in the coefficient of static friction of cleaned metal cans. Also, the compositions disclosed in the references may not be sufficiently oven-stable, that is, they may volatilize in the dry-off oven during a line stoppage and, consequently, exhibit poor mobility.
The use of various waxes, such as branched hydrocarbon waxes, carnauba waxes and the like, for improving storage stability of aqueous-based metallic surface coatings is well known in the art. Also, the use of these waxes in metal container coatings, for example, varnish and basecoat compositions, is known to enhance mobility of the coated containers as they are conveyed through trackwork in the manufacturing facilities, as well as in beverage filling lines. The application of such waxes in metal container washing processes as mobility enhancers, however, is not known.
It has been found that the addition of a mobility enhancing composition comprising one or more particular waxes in combination with at least one surfactant to disperse the wax in aqueous media provides a notable improvement in the reduction of the coefficient of static friction of cleaned metal containers, thereby improving mobility of the cans as they are conveyed along trackwork in a manufacturing facility. Such mobility enhancing compositions are dry off oven stable and provide a substantially water-break-free metal surface.